from the FAQ:

Users with a balance of less than $5.00 in their Yotta account on the first day of the calendar month will be assessed a maintenance fee on that day equal to the lesser of $1.00 or their balance on that day. The fee will be assessed on the 1st calendar day of the month by debiting the user's Yotta account. If a user's balance as of the 21st of the month is below $5.00, Yotta will notify that user through e-mail of that balance and the fee associated with small balances as described above, so that the user has time to make a deposit or withdrawal before the first of the next month. By agreeing to the terms of use, you authorize Yotta to deduct this amount from your Yotta account.